1. Field of the Invention
The present invention relates to polyester foamed articles prepared by using aliphatic polyesters with biodegradability and sufficiently high molecular weights and specific melt properties for practical use. More particularly, the present invention relates to a method for producing foamed articles by extrusion or mold forming using expandable particles (beads) and foamed articles produced thereby.
2. Discussion of the Background
Recently, although plastic foamed articles characterized by their lightness, elasticity, moldability, etc. are being used mainly as packaging materials, cushioning materials, etc., there is a great community problem in that the resulting waste of the large amounts of plastics used in these materials are hard to dispose of and can pollute rivers, oceans, and soil. To prevent such pollution the development of biodegradable plastics has been desired; for example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of general-purpose plastics and starch, a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a heat decomposition temperature close to its melting point and a raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer has defects in molding properties, and is greatly limited in its range of application.
On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because polymeric material sufficient enough to obtain practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point of 62.degree. C. and a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of glycolide and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as packaging materials, cushioning materials and the like because their decomposition temperatures are close to their melting point and they have defects in their molding properties.
Materials generally used for molding foamed articles are polystyrene or polyethylene. These materials are mixed with foaming agents and then formed to the foamed articles by extruding or molding in a mold.
A material such as styrene polymer containing 0.5.about.40 parts by weight of a volatile foaming agent, such as propane, butane, pentane, methyl chloride, or dichlorofluoromethane, are known as expandable beads. When heated to a temperature not lower than their softening point, these expandable beads become pre-expanded beads in which a large number of small cells have been generated. A method of producing foamed articles is known, according to which a closed-but-nonairtight type mold having a large number of small holes in its walls and a configuration which is the same as that of the final product is filled with the above pre-expanded beads, and a heating medium, such as steam, is sprayed onto the beads through the above-mentioned small holes to heat them to a temperature not less than their softening point, thereby causing the beads to be expanded and fused to each other so as to produce a porous-styrene-polymer foam having exactly the same shape as the above mold.
On the other hand, the foamed polystyrene articles generate combustion heat as high as 9,500 kcal/kg, so a fear to harm an incinerator is accompanied.
Usually, polystyrene is used in the production of such expandable beads, and even polyethylene terephthalate, which is a condensation product of ethylene glycol and a terephthalic acid (inclusive of dimethylterephthalate), a high-molecular-weight polyester (which means here one having a number average molecular weight of at least 10,000) that is being applied to general packing uses, is not used in this field, and there has been no reported attempt to impart biodegradability to such materials.
Thus, there has been absolutely no technical concept of attempting to put to practical use expandable beads formed from a biodegradable aliphatic polyester based on an aliphatic-type dicarboxylic acid.
One of the reasons for the absence of a concept of putting such foamed articles into practical use is that even in a crystalline condition, most of such aliphatic polyesters have a melting point not higher than 100.degree. C. although special production conditions and physical properties are required with regard to the above expandable beads. Further, they have poor stability when melted. What is more important, the properties of these aliphatic polyesters, particularly the mechanical properties thereof, such as tensile strength, are markedly poor even with a number average molecular weight which is of the same level as the above-mentioned polyethylene telephthalate, so that it has been difficult to even imagine the possibility of obtaining from such materials a molding which must exhibit high strength.
Also it is to be assumed that this is partly attributable to the fact that studies toward the possibility of attaining an improvement in physical properties by increasing the number average molecular weight of such aliphatic polyesters have not progressed to a satisfactory degree yet due to the poor thermal stability of such polyesters.
It is an object of this invention to provide polyester foamed articles which contain an aliphatic polyester as mentioned above, which have a sufficiently high molecular weight for practical uses, which excel in mechanical properties such as thermal stability and tensile strength, and which exhibit biodegradability as a disposal means, that is, which can be decomposed by microorganisms or the like, thereby facilitating the disposal thereof after use.